Transformers cores are typically made of layers of magnetic steel in order to reduce eddy current effects. One approach has been to manufacture stacked cores in which Silicon Steel is cut into lengths and stacked on top of one another to form a stack of laminated steel. Typically stacks are arranged in core configurations e.g., a FIG. 8 configuration in which the stacks are intertwined at their corners. This approach works adequately when dealing with Silicon Steel sheets that are typically of the order of 10 mil thick. However the downside is two-fold. Firstly, the machinery for cutting and stacking the sheets is extremely expensive and due to the repetitive cutting actions and back and forth movements, is prone to frequent failure and requires a high degree of maintenance. Secondly, the flat stack technology becomes very time consuming and costly when using thinner material.
More recently materials such as amorphous metal that is of the order of 1 mil thick and nano-grain steel that is of the order of 2 mil thick have become available. The flat stack technology discussed above therefore does not provide a satisfactory solution to making transformer cores using these materials. Thus, although these materials display lower losses, the flat stack manufacturing process does not lend itself to making cores from the material.
An alternative method of making cores that has also been used involves the winding of core material onto a reel. In the case of smaller wound cores the transformer conductor is usually subsequently wound through the window using a bobbin to form coils on the core, or in some instances the ferromagnetic core material may be wound through the coil. In larger wound core units the ferromagnetic material is cut to varying lengths (laminations) and wound into a generally square shape with gaps in the core that can be opened (unlaced) to allow coils to be landed upon the core at which point the core can be closed (re-laced) to complete the magnetic circuit.
Extant techniques for manufacturing wound cores composed of amorphous metal are, for example, described in U.S. Pat. Nos. 5,285,565; 5,327,806; 5,063,654; 5,528,817; 5,329,270; and 5,155,899, which describe ribbon winding, lamination cutting, lamination stacking, lamination winding, annealing, and coating the edge of the core.
The prior art wound cores, however, typically have a generally rectangular shape with a joint at one end that can be opened (unlaced) to allow the landing of a coil that has been wound separately. Once the coil has been landed the core must be closed (re-laced).
Irregularities in the laminations can prevent the joint overlaps from matching perfectly.
Also, the winding of the amorphous core into a generally rectangular shape can detrimentally impact the performance since stresses are introduced when forming the corners of the core.
Additional problems are encountered specifically when dealing with amorphous metal as the magnetic core material. Amorphous laminations are thin, ranging from 0.001 to 0.0011 inches in thickness. Amorphous metal also lacks the structural integrity of silicon steel, displaying instead the floppiness of wet tissue paper even though it has quite high tensile strength. By comparison, silicon steel has much greater structural integrity than amorphous metal so that the silicon steel core is capable of retaining its shape once wound.
Thin materials such as amorphous metal and nano-grain steel also require 5 to 10 times more layers to build up the core, requiring a longer winding process and more difficulties with unlacing and re-lacing of the core in order to land the coils on the core.
Amorphous metal also becomes quite brittle once annealed, making this core manufacturing process quite complex when compared to the core manufactured from silicon steel. The brittleness of annealed amorphous metal leads to inevitable breakage and flaking when unlacing and re-lacing an amorphous core.
It will therefore be appreciated that a construction method for amorphous and nano-grain cores that eliminates lamination damage and breakage, reduces stress within the core, reduces the time to assemble the transformer, and the time required to wind the core would be very valuable. In particular, it would allow the realization of the potentially low losses offered by amorphous metal and nano-grain steel.